The present invention relates to an instrument for maintaining the distance between a tool and a workpiece to be processed constant, in particular, for keeping the distance constant between a cutting torch of a flame cutting machine and a workpiece, with a distance change-detecting capacitor formed by a sensing electrode preferably connected to the torch and by the workpiece surface and which is connected to a first phase control system-building block with an incorporated set point-actual value comparing device and a torch adjusting device.
An arrangement of the above type is disclosed in German Preliminary Application DE-OS No. 27 44 362. With this capacitive height adjusting instrument, a considerable control accuracy is attained with the use of a phase control system since even the smallest capacity variations lead to relatively large output signal changes and the torch height adjusting device can be accurately adjusted. Practical use has demonstrated, however, that the temperatures (20.degree.-60.degree. C.) occurring near the height control during flame cutting lead to a mistuning of the phase control system as a result of which the desired control accuracy is not always guaranteed.